Problem with DTMF MT8870D ouput stays high with power ON-OFF cycle

Discussion in 'The Projects Forum' started by Pankajvidya, Sep 17, 2014.

  1. Pankajvidya

    Thread Starter New Member

    Sep 17, 2014
    1
    0
    Hi Folks,
    I'm working with MT8870 to turn on the water pump using cell phone. The circuit works fine but there is small issue. Here are few things about my circuit,
    1. The output (Pin. 11) stays latched which helps to hold relay on continuously until it receive another tone. Relay operated by using pin no.11 (Key "9" activate the Relay), 13K resistor & 2N3904 transistor.
    2. This operation is as per expectation.
    3.The pin 10 is connected to +5 VDC and pin 15 left open.
    Here is issue:
    When I power OFF-ON the circuit within 1-2 minute with activated relay, the relay stays latched but if I power OFF - ON the circuit with 20-30 min time gap the relay stays de-activated.
    I don't want to keep the relay in latch condition even after power OFF-ON cycle with any time gap. The MT8870D output (Pin 11 to Pin 14) should be go low with the power OFF-ON cycle.
    Please advice.
    Regards,
    Pankaj
     
  2. rogs

    Active Member

    Aug 28, 2009
    279
    37
    The MT8870 has no 'power on reset' state. You cannot guarantee the status of any of the outputs, after power is applied, until a valid DTMF code has been received.
    If pin 10 is held high, then the output latches will power up in an active, but unknown state. Your observations so far are purely by chance... the state is random.

    See the note attached here for a possible hardware solution:

    http://forum.allaboutcircuits.com/t...of-telephone-dtmf-mt8870ce.34741/#post-217156

    EDIT: I've just realised that circuit was drawn to source current to LEDs, fitted with current limit resistors.
    For your application, you will need to pull each output 'Lo' (to Ov) with an individual resistor (normally 100k).
    That is because pin 10 powering up 'Lo' forces the output pins into a tri-state Hi-Z state until the first valid code 'sets' the bi-stable, which forces pin 10 'Hi', and activates the output latches.
     
    Last edited: Sep 18, 2014
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